For better understanding the terminology used in the present description and principles of structure of optical systems in general, it would be advantageous to make some short introduction into the field of optical objectives.
An objective is an optical system or a part thereof that faces an object of observation or photographing and that creates a real image of the object turned 180° with respect to the object. Depending on the types of optical elements, objectives can be divided into lens-types, mirror-types, mirror-lens-types, and kinoform-type objectives. Most popular are lens-type objectives that are capable of acquiring various characteristics due to increase in the number of component lenses.
Photographic objectives or similar objectives of motion-picture cameras, TV cameras, night-vision instruments, and objectives used in television generally create reduced images of remote objects on a layer of a photosensitive material or on photoreceivers, e.g., TV picture tubes, matrices or linear photoreceivers, or photocathodes of optoelectronic devices. The scale of an image is proportional to a focal distance f′ of the objective, while illumination intensity is inversely proportional to a second power of a diaphragm number K, which is an f′/D ratio where D is a diaphragm of an inlet pupil of the objective. A value of 1/K is known as an aperture ratio. The limit value of the diaphragm number that allows correction of aberrations is K=0.5. A majority of existing objectives have K within the range of 3>K≧1.2. Photographic resolution capacity Nf of photo and motion-picture objectives depends on aberrations, as well as on resolution capacity Nc of the photosensitive layer of the reproducing medium and can be calculated with the use of the following approximated formula: 1/Nf=1/N0+1/Nc, wherein N0 is a visual resolution capacity of the objective. In a lens system, aberration is an error resulting from a failure of light rays from one point to converge to a single focus. A part of a space or surface the points of which are reproduced by the objective with a required quality is characterized by an angular field, i.e., a flat angle 2ω that corresponds to a solid angle that is coaxial with the optical axis and has the apex in the center of the inlet pupil of the objective. Angular field of modern photo cameras is normally within the range of 40° to 70°, while in aerophoto cameras this angle may reach 140°.
A special group is pancreatic objectives which are also known as zoom lenses, the focal distance of which can be smoothly adjusted in a wide range by displacing separate lenses or groups of lenses along their optical axis. The number of lenses in such objectives may be as high as 30 or more. Such objectives are used, e.g., in transmission TV cameras, video cameras, and photo cameras. A ratio between the maximal and minimal focus distances may reach 40, or more. For decrease of optical losses, modern objectives are provided with anti-reflective coatings.
Normally, conventional wide-angle photographic objectives or lens systems have big dimensions, i.e., a lengthy objective, and therefore are inconvenient for use and storage. Another characteristic feature of a wide-angle photographic lens system is an increased diameter. This not only increases the overall radius and hence the dimensions of the lens system but also significantly increases the weight of the objective as a whole.
There exist a large number of wide-angle photographic lens systems of different types, e.g., conventional photographic lens systems for photocameras, image projecting lens systems, wide-field lithography systems, etc.
For example, U.S. Pat. No. 4,188,092 issued in 1980 to Kikuo Momiyama describes a retrofocus type lens for a photocamera having an angle of view at least 75° and F number 1:2.0. The lens includes a first lens group of a divergent type, a second lens group of a convergent type, and a third lens group of a convergent type. The first lens group includes in the order stated a positive meniscus lens, a negative meniscus lens, a positive meniscus lens, and a negative meniscus lens. The second lens group includes a positive lens, which is either a single lens or consists of a positive lens, and a negative lens cemented to each other and with a front convex face directed toward an object to be photographed. The third lens group includes a positive lens having a rear convex face directed toward an image of the object, a biconcave lens with its front surface radius smaller than its rear surface radius, a positive meniscus lens with a convex surface facing the image, and a positive lens. The biconcave lens and the positive meniscus lens are respectively replaceable with cemented doublet lenses. The lens system is characterized in that the first lens group includes meniscus lenses arranged in the order of positive, negative, positive and negative lenses, and particularly in that the third positive meniscus lens serves effectively to correct chromatic distortion aberration and chromatic coma aberration.
Another example, e.g., U.S. Pat. No. 6,084,719 issued in 2000 to Saburo Saguwara, et al. discloses a projection optical system that includes a first lens unit in which negative lenses included therein are larger in number than positive lenses included therein, and a second lens unit in which positive lenses included therein are larger in number than negative lenses included therein. In this projection system, design parameters are determined such that an off-axial principal ray intersects an optical axis at a point between the first lens unit and the second lens unit, and telecentricity is made on the second conjugate point side. The second lens unit includes a negative lens of meniscus form convex toward the second conjugate point side and a positive lens whose both surfaces are convex.
A common problem associated with wide-angle lens systems of the types described above as well as with other conventional wide-angle lens systems is that an increase in the aperture ratio of the lens system, widening of the field of observation, and improvement in resolution capacity of the optical system require an increase in the lens diameter. However, such an increase leads to more noticeable aberrations, and in order to solve the aberration problem, it is necessary to introduce into the system new optical elements. However, Increasing the number of lens elements to overcome the above-described drawbacks degrades the performance of the lens system due to adverse effects such as flare. All this significantly increases the manufacturing cost and the cost of the products.
Attempts have been made, however, to solve the above problems and to improve conventional wide-angle lens systems, e.g., by increasing the amount of optical elements.
For example, U.S. Pat. No. 5,790,324 issued in 1998 to Cheon-Ho Park describes a wide-angle photographic lens system in which improvement in optical characteristics is achieved at the expense of complexity, increased weight, and increased cost. More specifically, the aforementioned lens system consists of seven lens elements, including combined lens elements.
One of the latest patents in this field, i.e., U.S. Pat. No. 6,545,824 issued in 2003 to Sensui Takayuki, discloses a significantly improved lens optical system, in which the number of lens elements is reduced to five along with a twice shorter length of the system as a whole. Nevertheless, while preserving the traditional structure, the lens optical system of U.S. Pat. No. 6,545,82 still remains large in size, heavy in weight, and complicated in structure. These problems will always remain until a wide-angle lens system is designed on traditional principles of a wide-lens system architecture.
Development of optical fiber systems, light-emitting diodes and laser diodes, systems of management, control, and conversion of light beams in optical communication systems, etc. gave impetus to developing new and efficient microoptical systems such as microlenses, microobjectives, collimators, etc. In principle of their operation and structure, the aforementioned optical elements are the same as respective traditional optical lenses, objective, collimators, etc., but are intended for working with optical beams of small diameters, e.g., from several tens of microns to several millimeters. Miniaturization of optical elements to the level of current microlenses led to very stringent requirements with regard to manufacturing accuracy and narrowed the allowable tolerances, e.g., on optical surfaces, to nanometric level. Recent success in this technology made it possible to produce microoptical lenses with very accurate aspherical surfaces.
One of the fields in which microoptics may find application is photolithography. For example, U.S. Pat. No. 6,016,185 issued to Elmar Cullman, et al. discloses an apparatus and method for photolithographic exposure of a substrate including an illumination source for providing light for producing an image on the substrate, a mask including a pattern for projection onto the substrate, a microlens assembly for projecting the light through a plurality of microlens channels onto the substrate and an actuator for moving the microlens assembly in a plane parallel to the mask and the substrate for suppressing interference effects. The above system is intended for fixed-scale (1:1) projection of an object onto an image plane. In other words, the microlenses of this system are assembled into an array that forms an objective for parallel transfer of an image without any reduction or magnification of the image.
A series of inventions made by Stephen Daniell (see, e.g., U.S. Pat. No. 6,721,101 issued in 2004) relates to the use of a microlens optical system for obtaining a 3—D image in the observer's sight. This technique is based on the principle of creation of parallax between the “left” and “right” images, which is perceived by the observer as a stereo effect.
The arrays used in the above inventions can be divided into two categories. The arrays of the first type uses air as a low-index material. Such arrays may be used, for example, in illuminated displays of electronic image detection, machine vision, and real-time 3D video capture. Arrays of the second use a fluoropolymer as a low-index material, and convey a great preponderance all incident light to the image plane.
More specifically, the system of U.S. Pat. No. 6,721,101 (as well as the systems of all other inventions of Stephen Daniell) is an assembly of two array substrates, which in an overlapped state possess better optical characteristics than a single array substrate. From the optical point of view, this system functions as follows: an object located at a finite distance from the observer is converted by the overlapped arrays into an infinitely located image which is observed with the maximum possible angle of observation. This allows the observer, who is located on the symmetry axis of a display, to clearly see on this display two independent images of one object with the left and the right eyes.
In reality, the Daniell's system does not widen the angle of observation for the observer but rather creates a virtual effect of stereo vision. In this system, the second and third surfaces of the array work as a separator of angles of incidence of light, i.e., starting from angle that exceeds a predetermined value, the light does not pass through the system but is reflected on the principle of total inner reflection, e.g., to the right eye, while the light incident at smaller angles passes through the system, is focused on the last flat plane of the lens system, and returns to the left eye.
Although the Daniell's system cannot be used for widening a real angle of observation and merely redistributes and divides the optical path of light that passes through the system for stereo effect, this system is a good example of a two-array assembly for optical purposes. The use of a sandwich composed of two overlapped film-like or plate-like arrays makes it possible to significantly reduce the geometrical dimensions of the lens system.